<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="botton" value="点我" onclick="on()"><br>
<input type="botton" value="再点我" id="btn">



<script>

  function on(){
    alert("我被点了");
  }

  document.getElementById("btn").onclick = function (){
      alert("我被点了");
  }
</script>
</body>
</html>